OBJECT ORIENTED PROGRAMMING USING C++

ISBN 0-321-33025-0

Abstract Data Types and Encapsulation Concepts

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Chapter 11 Topics

• The Concept of Abstraction• Introduction to Data Abstraction• Design Issues for Abstract Data Types• Language Examples• Parameterized Abstract Data Types• Encapsulation Constructs• Naming Encapsulations

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The Concept of Abstraction

• An abstraction is a view or representation of an entity that includes only the most significant attributes

• The concept of abstraction is fundamental in programming (and computer science)

• Nearly all programming languages support process abstraction with subprograms

• Nearly all programming languages designed since 1980 support dataabstraction

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Introduction to Data Abstraction

• An abstract data type is a user-defined data type that satisfies the following two conditions:– The representation of, and operations on,

objects of the type are defined in a single syntactic unit

– The representation of objects of the type is hidden from the program units that use these objects, so the only operations possible are those provided in the type's definition

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Advantages of Data Abstraction

• Advantage of the first condition– Program organization, modifiability

(everything associated with a data structure is together), and separate compilation

• Advantage the second condition – Reliability--by hiding the data

representations, user code cannot directly access objects of the type or depend on the representation, allowing the representation to be changed without affecting user code

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Design Issues

• A syntactic unit to define an ADT• Built-in operations

– Assignment– Comparison

• Common operations– Iterators– Accessors– Constructors– Destructors

• Parameterized ADTs

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Language Examples: Ada

• The encapsulation construct is called packages– Specification package (the interface)– Body package (implementation of the entities

named in the specification)• Information Hiding

– The representation of type appears in a part of the specification called the private part

• More restricted form with limited private types– Define the ADT as a pointer and provide the pointed-

to structure’s definition in the body package

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An Example in Adapackage Stack_Pack is

type stack_type is limited private;max_size: constant := 100;function empty(stk: in stack_type) return Boolean;procedure push(stk: in out stack_type; elem:in Integer);procedure pop(stk: in out stack_type);function top(stk: in stack_type) return Integer;

private -- hidden from clientstype list_type is array (1..max_size) of Integer;type stack_type is record

list: list_type;topsub: Integer range 0..max_size) := 0;

end record;end Stack_Pack

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Language Examples: C++

• Based on C struct type and Simula 67 classes

• The class is the encapsulation device• All of the class instances of a class share a

single copy of the member functions• Each instance of a class has its own copy of

the class data members• Instances can be static, stack dynamic, or

heap dynamic

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Language Examples: C++ (continued)

• Information Hiding– Private clause for hidden entities– Public clause for interface entities– Protected clause for inheritance

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Language Examples: C++ (continued)

• Constructors:– Functions to initialize the data members of

instances (they do not create the objects)– May also allocate storage if part of the object

is heap-dynamic– Can include parameters to provide

parameterization of the objects– Implicitly called when an instance is created– Can be explicitly called– Name is the same as the class name

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Language Examples: C++ (continued)

• Destructors– Functions to cleanup after an instance is

destroyed; usually just to reclaim heap storage– Implicitly called when the object’s lifetime ends– Can be explicitly called– Name is the class name, preceded by a tilde (~)

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An Example in C++class stack {

private:int *stackPtr, maxLen, topPtr;

public:stack() { // a constructor

stackPtr = new int [100];maxLen = 99;topPtr = -1;

};~stack () {delete [] stackPtr;};void push (int num) {…};void pop () {…};int top () {…};int empty () {…};

}

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Evaluation of ADTs in C++ and Ada

• C++ support for ADTs is similar to expressive power of Ada

• Both provide effective mechanisms for encapsulation and information hiding

• Ada packages are more general encapsulations

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Language Examples: C++ (continued)

• Friend functions or classes - to provide access to private members to some unrelated units or functions– Necessary in C++

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Language Examples: Java• Similar to C++, except:

– All user-defined types are classes– All objects are allocated from the heap and

accessed through reference variables– Individual entities in classes have access

control modifiers (private or public), rather than clauses

– Java has a second scoping mechanism, package scope, which can be used in place of friends

• All entities in all classes in a package that do not have access control modifiers are visible throughout the package

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An Example in Javaclass StackClass {

private:

private int [] *stackRef;

private int [] maxLen, topIndex;

public StackClass() { // a constructor

stackRef = new int [100];

maxLen = 99;

topPtr = -1;

};

public void push (int num) {…};

public void pop () {…};

public int top () {…};

public boolean empty () {…};

}

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Language Examples: C#• Based on C++ and Java• Adds two access modifiers, internal and

protected internal• All class instances are heap dynamic• Default constructors are available for all

classes• Garbage collection is used for most heap

objects, so destructors are rarely used• structs are lightweight classes that do

not support inheritance

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Language Examples: C# (continued)• Common solution to need for access to

data members: accessor methods (getter and setter)

• C# provides properties as a way of implementing getters and setters without requiring explicit method calls

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C# Property Example public class Weather {

public int DegreeDays { //** DegreeDays is a propertyget {return degreeDays;} set {degreeDays = value;}

}private int degreeDays;...}

...Weather w = new Weather();int degreeDaysToday, oldDegreeDays;...w.DegreeDays = degreeDaysToday;...oldDegreeDays = w.DegreeDays;

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Parameterized Abstract Data Types

• Parameterized ADTs allow designing an ADT that can store any type elements

• Also known as generic classes• C++ and Ada provide support for

parameterized ADTs• Java 5.0 provides a restricted form of

parameterized ADTs• C# does not currently support

parameterized classes

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Parameterized ADTs in Ada• Ada Generic Packages

– Make the stack type more flexible by making the element type and the size of the stack generic

genericMax_size: Positive;type Elem_Type is Private;package Generic_Stack is …function Top(Stk: in out StackType) return Elem_type;…end Generic_Stack;

Package Integer_Stack is new Generics_Stack(100,Integer);Package Float_Stack is new Generics_Stack(100,Float);

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Parameterized ADTs in C++• Classes can be somewhat generic by

writing parameterized constructor functions

template <class type>class stack {…stack (int size) {stk_ptr = new int [size]; max_len = size - 1;top = -1;

};…

}

stack stk(100);

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Encapsulation Constructs

• Large programs have two special needs:– Some means of organization, other than simply

division into subprograms– Some means of partial compilation (compilation

units that are smaller than the whole program)• Obvious solution: a grouping of

subprograms that are logically related into a unit that can be separately compiled (compilation units)

• Such collections are called encapsulation

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Nested Subprograms• Organizing programs by nesting

subprogram definitions inside the logically larger subprograms that use them

• Nested subprograms are supported in Ada and Fortran 95

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Encapsulation in C

• Files containing one or more subprograms can be independently compiled

• The interface is placed in a header file• Problem: the linker does not check types

between a header and associated implementation

• #include preprocessor specification

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Encapsulation in C++

• Similar to C• Addition of friend functions that have

access to private members of the friend class

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Ada Packages

• Ada specification packages can include any number of data and subprogram declarations

• Ada packages can be compiled separately• A package’s specification and body parts

can be compiled separately

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C# Assemblies

• A collection of files that appear to be a single dynamic link library or executable

• Each file contains a module that can be separately compiled

• A DLL is a collection of classes and methods that are individually linked to an executing program

• C# has an access modifier called internal; an internal member of a class is visible to all classes in the assembly in which it appears

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Naming Encapsulations

• Large programs define many global names; need a way to divide into logical groupings

• A naming encapsulation is used to create a new scope for names

• C++ Namespaces– Can place each library in its own namespace and

qualify names used outside with the namespace– C# also includes namespaces

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Naming Encapsulations (continued)

• Java Packages– Packages can contain more than one class

definition; classes in a package are partialfriends

– Clients of a package can use fully qualified name or use the import declaration

• Ada Packages– Packages are defined in hierarchies which

correspond to file hierarchies– Visibility from a program unit is gained with the with clause

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Summary

• The concept of ADTs and their use in program design was a milestone in the development of languages

• Two primary features of ADTs are the packaging of data with their associated operations and information hiding

• Ada provides packages that simulate ADTs• C++ data abstraction is provided by classes• Java’s data abstraction is similar to C++• Ada and C++ allow parameterized ADTs• C++, C#, Java, and Ada provide naming

encapsulation